Many measurements of interest involve measuring the response of a sample to a light beam of a known wavelength as a function of the wavelength. The light source for many of these measurements is a tunable laser. Spectrographic systems in the mid-infrared range region (MIR) of the spectrum that utilize quantum cascade lasers are particularly useful in identifying the chemical composition of a specimen, since these lasers can be turned over a broad range of wavelengths in the region of the spectrum of interest. For the purposes of the present disclosure, MIR will be defined to be light between the wavelengths of 2.5 microns and 50 microns.
The tunable laser typically includes a gain chip, or other gain media, that amplifies light over a range of wavelengths and a cavity that includes a wavelength filter that determines the resonance wavelength of the filter. For example, in one type of external cavity laser, the cavity is formed by one facet of the gain chip and a diffraction grating that reflects light of the desired wavelength back to the gain chip. In this arrangement, the wavelength of the laser light can be varied by altering the angle at which the light leaving the gain chip strikes the grating.
The laser tuning range is determined by details of the external cavity and the gain as a function of wavelength of the gain chip. It would be advantageous to measure the gain profile as a function of wavelength for the gain chip without having to integrate the gain chip into an external cavity laser.